MNG (Multiple Network Graphics) Format Version 19960928

If an IHDR-IEND sequence or a NONE chunk is not immediately preceded by a DEFI or DEFN chunk, then "DEFI 0" is implied, unless the "ok_to_discard" field of the MHDR chunk is set.

2.1.10. CLRB and NOCL Clear the display

The NOCL chunk turns off the "clear to background" mode.

The CLRB and NOCL chunks are empty.

2.1.11. FRAM and ENDF Compose a frame

The FRAM chunk is empty.

The LOCA chunk can be used to specify the placement of each image within the frame. If the images are transparent or do not cover the entire frame, as defined by the max_frame_width and max_frame_height fields of the MHDR chunk, they are composited against the background defined by the BACK chunk, or against an application-defined background, if the BACK chunk is not present.

Viewers are expected to ignore the frame_duration value while inside the {FRAM, ENDF} pair and display all of the images at once, if possible, or as fast as can be managed. The frame_duration value is the desired minimum time to elapse from the beginning of displaying the first image of the frame until the beginning of the next image after the entire frame.

When images in a frame overlap, viewers are expected to composite the later images against the partially completed frame that includes all earlier images.

The ENDF chunk marks the end of a composite frame. This chunk is empty. For every FRAM chunk, there must be a corresponding ENDF chunk later in the MNG datastream, and for every ENDF chunk, there must be a corresponding FRAM chunk earlier in the MNG datastream.

The FRAM chunk is exactly equivalent to the series

CLRB
NOCL
dURA 0

CLRB
dURA (value of duration when the FRAM chunk was encountered).

2.1.12. CLON Clone an image

2 bytes: image_id (unsigned integer) identifier of the
         basis image to be cloned.

2 bytes: clone_id (unsigned integer) identifier to be
         to be given to the clone (new copy) to serve
         as the image_id of the new image.

Subsequent chunks can modify, show, or discard the clone or its associated LOCA, INHR, or SBYK data without affecting the basis image, or they can modify, show, or discard the basis image or its associated data without affecting the clone.

The CLON chunk does not imply SHOW.

2.1.13. SVCF Save composited frame

2 bytes: image_id (unsigned integer)
4 bytes: width (unsigned integer)
4 bytes: height (unsigned integer)
1 byte:  bit_depth (unsigned integer)
1 byte:  color_type (unsigned integer)
1 byte:  compression_method (unsigned integer)
1 byte:  filter_type (unsigned integer)
1 byte:  interlace_type (unsigned integer)

2.1.14. SHOW Show images

2 bytes: first_id (unsigned integer) 
2 bytes: last_id  (unsigned integer) 

An instance of each image will be displayed at the location specified by the LOCA chunk, if the LOCA chunk is present. When the LOCA chunk omitted, each image is displayed at the same location as its previous location. When the LOCA chunk is used in the delta form, which will usually be the case, each image will be displaced from its previous position by the values given in the LOCA chunk.

It is not necessary to follow an IHDR-IEND or DHDR-DEND sequence with a SHOW chunk. Such images are always displayed if they are within clipping boundaries of the frame, unless the sequence was preceded by the DEFN chunk.

2.1.15. DISC Discard images

The chunk contains a sequence of zero or more two-byte image identifiers. The number of images to be discarded is the the chunk's data length, divided by two.

2 bytes: discard_id (unsigned integer) image identifier
         that can be discarded.  All information
         pertaining to the corresponding image can be
         disarded and the identifier can be reused by
         a DEFI chunk.
etc.

When an image is discarded, any LOCA, INHR, or SBYK data associated with it is also discarded.

The appearance of an image_id in the discard_id list, when no such image has been stored, or when the image has already been discarded, should not be treated as a fatal error.

Discarding the background image is not permitted.

2.1.16. SAVE Save decoder state

It appears after the set of chunks that define the decoder state that must be restored upon encountering a SEEK chunk.

Only one instance of the SAVE chunk is permitted in a MNG datastream. It is not allowed anywhere after the first SEEK chunk.

2.1.17. SEEK Seek point

n bytes: previous (number of bytes since the
         previous SEEK chunk) If previous==0,
         then the number of bytes is unspecified.

n bytes: next (number of bytes to the next SEEK
         chunk) If next==0, then the number of
         bytes is unspecified.

n is the length of the SEEK chunk, divided by two.
         n must be either 4 or 8.

When n is eight, 32-bit machines will have to interpret "previous" as a set of two integers, the first representing the number of complete 4G blocks and the second (the last four bytes of "previous") as the remainder, and will have to treat "next" similarly.

"Previous" and "next" are measured from the first length byte of one SEEK chunk to the first length byte of another SEEK chunk.

Applications are allowed to forget everything preceding the SEEK chunk, except for data appearing in the MHDR chunk and anything appearing ahead of the SAVE chunk, if the SAVE chunk is present.

The SEEK chunk is not permitted within the scope of a {LOOP, ENDL} pair. If a decoder encounters a SEEK chunk while any loop is active, either as a result of an illegal SEEK chunk appearing inside a loop or as the result of skipping corrupted data, all display loops must be immediately terminated.

Multiple instances of the SEEK chunk are permitted. The SEEK chunk must not appear prior to the SAVE chunk, if the SAVE chunk is present.

2.1.18. LOOP, ENDL Define a loop

1 byte: start_loop_level (unsigned integer)
1 byte: loop_effect (unsigned integer)
        0: Execution of the loop might modify or relocate
           (via the LOCA chunk) basis images.
        1: Execution of the loop might modify or relocate
           basis images, but upon completion of the loop,
           all basis images have been restored to their
           initial state and location.
        2: Execution of the loop does not modify or
           relocate any basis images.
4 bytes: repeat_count (unsigned integer) range 0 to
           2^31-1

When the LOOP chunk is present, an ENDL chunk with the same loop_level must be present later in the MNG datastream. Loops can be nested. Each inner loop must have a higher value of start_loop_level than the loop that encloses it.

If repeat_count is zero, the loop is done zero times. Upon encountering a LOOP chunk with repeat_count==0, decoders simply skip chunks until the matching ENDL chunk is found, and resume processing with the chunk immediately following it.

It is the responsibility of the encoder to make sure that the assertions made by the loop_effect field are true. Note that the loop_effect field says nothing about the appearance of the display at the end of the loop. It describes the state of the basis images at the end of each iteration of the loop. When loop_effect==1, all iterations of the loop are identical, and a viewer could choose to store copies of the frame buffer for redisplay. This is also true when loop_effect=2; furthermore, if the user has escaped from the interior of the loop it is safe to resume processing with the first chunk after the ENDL chunk without having to skip to a SEEK chunk.

The ENDL chunk ends a loop that begins with the LOOP chunk. It contains a single one-byte field:

1 byte: end_loop_level (unsigned integer) range 0 to 255

When the ENDL chunk is present, a LOOP chunk with the same loop_level must be present earlier in the MNG datastream.

2.1.19. SYNC Synchronize display

2 bytes: sync_id (unsigned integer) sync identifier

2.1.20. NONE Define a blank image

The NONE chunk defines a blank image. The chunk data is the same as that of the PNG IHDR chunk:

4 bytes: width (unsigned integer)
4 bytes: height (unsigned integer)
1 byte:  bit_depth (unsigned integer)
1 byte:  color_type (unsigned integer)
1 byte:  compression_method (unsigned integer)
1 byte:  filter_type (unsigned integer)
1 byte:  interlace_type (unsigned integer)

It generates a rectangle with zeroes in all of the pixel samples, which represents a black rectangle, fully transparent if the color type is 4 or 6. If color_type==3, it also generates a PLTE of length 2^bit_depth, filled with zeroes.

2.1.21. NEED Resources needed

The NEED chunk contains a list of chunk names that the decoder must be prepared to encounter.

4 bytes: chunkname
etc.

The NEED chunk should be placed early in the MNG datastream, preferably immediately after the MHDR chunk. Viewers not recognizing critical chunk names in the list should abandon the MNG datastream or, if the unrecognized chunk name is ancillary, can display a warning or request user intervention.

2.1.22. INHR Global INHR chunk

2.1.23. SBYK Global SBYK chunk

2.2. Ancillary MNG chunks

2.2.1. dURA Duration

4 bytes: duration (unsigned integer), in ticks, using
         the tick length determined from ticks_per_second
         defined in the MHDR chunk.

2.2.2. gPLT Global Palette

The format of this chunk is identical to that of the [proposed] spAL PNG chunk, except that it lacks the "signature" and its zero-byte terminator. The chunk's contents are a zero-byte-terminated text string that names the palette, followed by a 4-byte gamma value, followed by a series of palette entries, each a ten-byte series, containing five unsigned integers:

    name:      n bytes (ASCII text)
    null byte  1 byte  (terminator)
    gamma      4 bytes

    red:       2 bytes (0 = black, 65535 = red)
    green:     2 bytes (0 = black, 65535 = green)
    blue:      2 bytes (0 = black, 65535 = blue)
    alpha:     2 bytes (0 = fully transparent,
                        65535 = fully opaque)
    frequency: 2 bytes (relative frequency of occurrence)
    ...

There can be any number of entries; a decoder determines the number of entries from the remaining chunk length after the null-terminated "name" string. This length not divisible by ten is an error. Entries must appear in decreasing order of "frequency".

The "name" (e.g. "256 color including Macintosh default", "256 color including Windows-3.1 default", "Browser Safe Palette") identifies the palette, which can permit applications or people to choose the appropriate one when more than one suggested palette appears in a MNG datastream. The "name" string must consist only of printable ASCII characters and must not have leading or trailing blanks, but can have single embedded blanks. There must be at least one and no more than 79 characters in the name. Names are case-sensitive. Decoders should filter out any nonprintable characters, especially the ESC character, in the "name" string before displaying it, to avoid possible security hazards.

The gamma field gives the value of gamma, times 100000, that is associated with the palette entries.

The red, green, and blue values are not premultiplied by alpha, nor are they precomposited against any background. A decoder can build a palette by compositing those palette entries against any background color or set of background colors that it chooses.

Each frequency entry is proportional to the approximate expected fraction of pixels in the images that are closest to that palette entry, without regard to any compositing against a background palette. The exact scale factor is chosen by the encoder, but should be chosen so that the range of individual values reasonably fills the range 0 to 65535. It is acceptable to artificially inflate the "frequency" values for "important" colors such as those in a company logo or in the facial features of a portrait. Zero is a valid value for frequency, meaning the color is "least important" or that it is rarely if ever used.

The palette uses 16 bits (2 bytes) per value regardless of the image bit depth specification. Decoders wishing to construct a palette with a smaller bit depth can accomplish this by scaling down the RGB entries, as described under "bit depth rescaling" in the PNG specification.

If the file gamma value for an image is different from the default gamma value from the MHDR chunk, decoders will need to gamma-correct the image samples before quantizing them to the gPLT palette.

Multiple gPLT chunks, with different names, are allowed in a MNG datastream. If present, they must appear prior to any IHDR or DHDR chunk to which they apply. The gPLT chunk can appear for any color type. When an image contains a PLTE or spAL suggested palette, the gPLT data takes precedence. The gPLT chunks should be placed ahead of the SAVE chunk, if the SAVE chunk is present, to ensure that the gPLT data is not lost when a SEEK chunk is encountered.

2.2.3. pSIZ Physical Image Size

4 bytes: x_dimension (unsigned integer),
         corresponding to max_frame_width, in
         micrometers.

4 bytes: y_dimension (unsigned integer),
         corresponding to max_frame_height.

When the pSIZ chunk is present and is recognized by the MNG viewer, the viewer is expected also to recognize and process the PNG pHYs and oFFs chunks encountered in images, even though these are ancillary chunks. When the pHYs chunk appears with unit_specifier==0, then that image should be scaled to obtain the desired aspect ratio by scaling the image height and leaving the image width fixed.

When the pSIZ chunk is not present, viewers are expected to ignore the pHYS and oFFs chunks, but MNG editors are expected to copy them as specified in the INHR chunk.

2.2.4. tEXt, zTXt, tIME Text, Time chunks

3. The PND format

No provision is made for storing a PND datastream as a standalone file. A PND datastream will normally be found as a component of a MNG datastream. Applications that need to store a PND datastream separately can wrap it in a MNG datastream consisting of the MNG signature, the MHDR chunk, a NONE chunk, the PND datastream, and a MEND chunk.

The decoder must have available a basis (decoded) image from which the original chunk data is known. The basis image can be the result of decoding a PNG or another PND datastream.

The new image is always of the same basic type (at present only PNG is defined) as the basis image.

The decoder must not have modified the pixel data in the basis image by applying output transformations such as gAMA or cHRM, or by compositing the image against a background. Instead, the decoder must make available to the PND decoder the unmodified pixel data along with the values for the gAMA, cHRM, and any other recognized chunks from the basis image datastream.

A PND datastream consists of a DHDR and DEND enclosing other optional chunks (if there are no other chunks, the decoder simply copies the basis image).

Chunk structure (length, name, CRC) and the chunk-naming system are identical to those defined in the PNG specification. Definitions of compression_type, filter_type, and interlace_type are also the same as defined in the PNG specification.

3.1. PND critical chunks

3.1.1. DHDR PND datastream header

The DHDR chunk must contain exactly 20 bytes:

2 bytes: image_id (unsigned integer)
         Identifies the basis image from which changes
         will be made.  This is also the image_id
         of the resulting modified image, which can be
         used as the basis image for a subsequent PND
         datastream.

1 byte: image_type
         0: Image type is unspecified.  An IHDR chunk
            must be present.
         1: Image type is PNG.   IHDR can be omitted
            if no IHDR fields are different from those
            in the basis image and delta_type is 0, 1,
            or 3, and if IHDR would otherwise have
            appeared immediately after DHDR.

1 byte: delta_type
         0: image replacement
         1: pixel addition, by samples, modulo
            2^bit_depth.
         2: alpha addition, by samples, modulo
            2^bit_depth.  Regardless of the color
            type of the basis image, the IDAT data
            are written as a grayscale image (color
            type 0) but the decoded samples are used
            as deltas to the alpha samples in the
            basis image.  The basis image must have
            (or be promoted to via the PROM chunk)
            color type 4 or color type 6.
         3: no change to pixel data

4 bytes: block_width (unsigned integer)

4 bytes: block_height (unsigned integer)

4 bytes: block_x_location (unsigned integer) measured
            in pixels from the left edge of the basis
            image.

4 bytes: block_y_location (unsigned integer) measured
            in pixels from the top edge of the basis
            image.

The block_width and block_height fields give the size of the block of pixels to be modified or replaced, and block_x_location and block_y_location give its location with respect to the top left corner of the basis image. The block must fall entirely within the basis image.

The block size and location fields are ignored when delta_type==3.

3.1.2. DEND End of PND datastream

The DEND chunk is empty.

3.1.3. PROM Promotion of basis image

1 byte: new color_type
1 byte: new bit_depth

When a grayscale or truecolor basis image is promoted to an image with an alpha channel, the alpha samples are initialized to 2^bit_depth-1 (fully opaque). When an indexed-color image is promoted, the alpha channel data is obtained from the basis image's tRNS chunk data, if present, or initialized to 2^bit_depth-1, if the tRNS chunk is not present or not recognized by the decoder.

The PROM chunk is not permitted to "demote" a basis image to an image with a lesser bit depth or from one with an alpha channel to one without an alpha channel.

The resulting image must not exceed max_bit_depth or max_samples_per_pixel from the MHDR chunk.

3.1.4. IHDR, PNG chunks, IEND

See the PNG specification for the format of the PNG chunks. The PNG datastream must contain at least IHDR and IEND but can inherit other chunk data from the basis image. Except for IDAT, any chunks appearing between IHDR and IEND are always treated as replacements or additions and not as deltas.

A gAMA, cHRM, or similar chunk existing in the basis image would not affect the pixel data inherited by this PND datastream because they are not used in decoding the pixel data. Applications are responsible for ensuring that the pixel values that are inherited from the basis image have not been transformed in any way after decompressing and unfiltering them.

When processing the tRNS chunk, if color_type==3 and PLTE is not supplied, then the number of allowable entries is determined from the number of PLTE entries in the basis image.

MNG viewers are expected to ignore the pHYS and oFFs chunks when the MNG pSIZ chunk is not present, but MNG editors are expected to recognized and copy the pHYS chunk, because it is a known chunk that is specified in the PNG specification, and to treat the oFFs chunk as an unknown chunk that will be handled as described in Paragraph 3.1.5, below.

The IHDR chunk can be omitted if all of the IHDR fields would be identical to those in the basis image, the image_type is 1, and the IHDR chunk would otherwise have appeared immediately after the DHDR chunk with no intervening PND chunks such as INHR. The decoder must treat this datastream as though the IHDR were present, immediately after the DHDR chunk, with all IHDR chunk data identical to that of the basis image. The IHDR chunk can also be omitted when no PNG chunks are present. When delta_type is 2, it is not necessary to include an IHDR chunk for the sole purpose of specifying that the IDAT is in grayscale format.

When the IHDR chunk is present and delta_type is nonzero, its width and height must match match those of the basis image. Also, when the IHDR chunk is present, the block height and width must match the height and width of the basis image, and the block location must be (0,0).

The PNG specification places ordering requirements on many chunks with respect to the PLTE and IDAT chunks. If neither of these two chunks is present, and the INHR chunk is not present, known chunks (always including all standard chunks described in the PNG specification) are considered to have appeared in their proper order with respect to the critical chunks. Unknown chunks are ordered as described in Paragraph 3.1.5, below. When the INHR chunk is present, then all chunks are considered to have appeared in the order given by the INHR chunk.

The IEND chunk can be omitted, if it would be the last chunk in the PND datastream before the DEND, or when no PNG chunks are present.

3.1.4.1. Image replacement

3.1.4.2. Image pixel deltas

The image color_type and bit_depth must match those of the basis image, and the basis image must have been derived from a PNG datastream or from a sequence of PND datastreams that depend upon a PNG datastream.

The compression method, filter method, and interlace method need not be the same.

[We might want to add a compression_type that uses deflate with the basis image data as a preset dictionary]

The IDAT chunk data contains a filtered and perhaps interlaced set of delta pixel samples. The delta samples are presented in the order specified by interlace method, filtered according to the filter method and compressed according to the compression method given in the IHDR chunk. The actual pixel values are calculated using the method defined in the delta_type field of the DHDR chunk.

When delta_type==1, an encoder calculates the new sample values from the samples in the basis image and those in the new image by subtracting the basis image samples from the new image samples, modulo 2^sample_bit_depth. When decoding the IDAT chunk, the new image bytes are obtained by adding the delta bytes to the basis image bytes, modulo 2^sample_bit_depth. This is similar in operation to the PNG SUB filter, except that it works by samples instead of by bytes.

When color_type==3, the deltas are differences between index values, not between color samples.

3.1.4.3. Image alpha deltas

The image bit_depth must match those of the basis image, but color type must be 0 (grayscale). The basis image must have an alpha channel or must have been promoted to a type that has an alpha channel. The compression method, filter method, and interlace method need not be the same.

3.1.4.4. No change to pixel data

When the delta_type==3 in the DHDR chunk, It is an error for IDAT to appear.

3.1.5. INHR Inherited Chunks

The INHR chunk contains a series of five-byte entries, each giving a chunk name and a rule.

4 bytes: chunkname
1 byte:  rule
         0: (Replace) Inherit all instances of
            the chunk in the basis image.  If a new
            instance is found in the PND stream,
            delete all inherited instances and replace
            them with the new instance or instances.
         1: (Append) Inherit all instances of the
            chunk in the basis image.  Append any
            new instances found in the PND stream.
         2: (Ignore) Ignore any instances of the
            chunk in the basis image.  Insert any
            new instances found in the PND stream.
         3: (Replace by Keyword) The chunk is one for
            which multiple instances are permitted,
            and the first field of the chunk is a
            null-byte terminated ASCII keyword or
            name (tEXt, zTXt, spAL, for example).
            Inherit all instances of the chunk from
            the basis image.  If a new instance is
            found in the PND stream, delete all
            inherited instances having the same
            keyword or name and replace them with
            the new instance or instances.
         4. (Special) Inheritence and placement of
            this chunk is governed by special rules
            that must be known to the PND decoder
            (IDAT, for example)
         5. (Select by Keyword) The chunk is one
            whose first field is a null-byte
            terminated ASCII keyword or name (tEXt,
            zTXt, spAL, for example).  Inherit only
            those instances named in SBYK chunks.
            Append any new instances found in the
            PND stream.
         6. (Reject by Keyword) The chunk is one
            whose first field is a null-byte
            terminated ASCII keyword or name (tEXt,
            zTXt, spAL, for example).  Inherit all
            instances except those instances named
            in SBYK chunks.  Append any new instances
            found in the PND stream.

The INHR chunk also gives a template for the arrangement of chunks within the resulting PNG datastream. The decoder can place them in any order, relative to their order in the basis image, that obeys the chunk-ordering rules for copy-safe chunks given in the PNG specification, but when it encounters unknown but copy-safe chunks, it should use the INHR chunk data as a template to ensure that they are properly placed.

The INHR chunk can be omitted. If no INHR chunk appears in a PND stream or is associated with its basis image, then data from the global INHR chunk, defined Paragraph 2.1.22, above, can be used if it is present. PND applications are expected to know the copying rules and order of placement of those chunks defined in the core specification for the image format corresponding to the DHDR image_type, regardless of whether an INHR chunk is present and whether those chunks are listed in it. In particular, the PNG IHDR and IEND chunks need not be listed in the INHR chunk, because the position of these chunks is already well-defined. PLTE (with rule==2) and IDAT (with rule==4) should be listed, however, because other chunks need to be located with respect to them. It is not an error for a chunkname to appear in the list, when that chunk does not appear in the basis image or in the PND datastream.

When an unknown chunk is found in the PND stream that appears in the basis image but is not listed in the INHR chunk, the decoder can copy the chunk into the same position relative to critical chunks that it occupied in the basis image. When an unknown chunk appears neither in the basis image nor in the INHR list, or when a chunk's rule==4 and the PND decoder does not know the special copying rules for the chunk in question, the decoder must treat it as an unknown chunk, ignoring it if it is ancillary, and abandoning the PND stream if it is critical.

The INHR chunk data itself is inherited, so an INHR chunk need only appear in the first of a sequence of PND datastreams that have the same image_id, if there are no changes. When a INHR chunk is inherited and also appears on the PND datastream, the entire list of chunk names and rules is replaced.

Applications that reconstruct a PNG datastream from a PND datastream and a basis PNG or PND datastream must not write the INHR chunk itself to the resulting PNG datastream, because the INHR chunk would not be recognized by a PNG decoder.

There can be only one instance of the INHR chunk in a PND datastream. The INHR chunk must appear before IHDR if it is present.

3.1.6. SBYK Select chunks by keyword

4 bytes:  (four ASCII bytes) chunk name
n bytes:  (ASCII string) Keyword

The keyword need not be terminated by a null byte; if it is, the null byte will be ignored. The keyword must follow the format of a tEXt keyword: It must consist only of printable ASCII characters and must not have leading or trailing blanks, but can have single embedded blanks. There must be at least one and no more than 79 characters in the name. Keywords are case-sensitive. Use caution when printing or displaying keywords (Refer to Security Considerations, Chapter 8, below).

Any chunks in the basis image having the same chunk name and keyword are selected (if INHR rule==5) or rejected (if INHR rule==6) for inclusion in the resulting PND image. It is a nonfatal error to select a {chunkname, keyword} pair that does not exist in the basis image. It is a fatal error to fail to include the chunkname in the INHR chunk if it appears in an SBYK chunk.

The SBYK chunk can be omitted. If no SBYK chunk appears in a PND stream or is associated with its basis image, then data from the global SBYK chunk, defined Paragraph 2.1.23, above, can be used if it is present.

The keyword, "purpose," or other identifying string must appear first in the chunk's data segment. The practice of putting a version identifying "signature" first in the data segment of unregistered experimental chunks will defeat this mechanism, so it is better to place such a "signature" after the keyword, if you wish to use the SBYK mechanism with your experimental chunk.

Applications that reconstruct a PNG datastream from a PND datastream and a basis PNG or PND datastream must not write the SBYK chunk itself to the resulting PNG datastream, because the SBYK chunk would not be recognized by a PNG decoder.

Multiple instances of the SBYK chunk are permitted in a PND datastream. Any instances must appear prior to the IHDR chunk, if the IHDR chunk is present.

3.2. PND ancillary chunks

3.2.1. fADE Fade in or out

   1 byte: (unsigned integer) fade_type
         0: fade out
         1: fade in
         2: fade in but don't change fully transparent
            pixels
   2 bytes: (unsigned integer) alpha_delta

When "fade_type==1", the value of alpha_delta is added to the alpha sample of every pixel in the basis image, but the result is not allowed to exceed the maximum alpha value for the image's bit depth.

When "fade_type==2", the value of alpha_delta is added to the alpha sample of any pixel in the basis image that has a non-zero value, and the result is not allowed to exceed the maximum alpha value for the image's bit depth.

If color_type==3, then the value of alpha_delta is added to or subtracted from the alpha values that were defined by the tRNS chunk data in the basis image, and the resulting values become the tRNS data exported to any subsequent image. The maximum alpha value for this color_type is 255, regardless of the bit depth.

If color_type is 0 or 2, the fADE chunk is ignored.

If IHDR is also present, the fADE chunk must appear before IHDR. The fade operation is performed on the image data after decoding the chunks between IHDR and IEND.

3.2.2. tEXt, zTXt, tIME Text, Time chunks

4. Retaining image data

The following information must be retained, for each image_id that is defined:

• The position of the image given by the LOCA chunk.
• The pixel data and the values associated with other recognized PNG chunks such as PLTE and gAMA, subject to the chunk copying rules and the INHR chunk.
• The contents of the INHR chunk and SBYK chunks associated with the image_id.

When the encoder knows that image data will not be needed by subsequent frames, it can make life easier for decoders by using the ok_to_discard field of the MHDR chunk or by using the DISC chunk.

When the encoder knows that all subsequent images will completely fill the display and do not use transparency, the background_source field of the MHDR chunk can be used to inform the decoder that it does not have to retain the background.

5. Decoder handling of fatal errors

When an error occurs within a image datastream, such as an unknown critical PNG chunk or a missing basis image where one was required, only that image should be abandoned and the associated image_id should be discarded.

MNG editors, on the other hand, should be more strict and reject any file with errors unless the user intervenes.

6. Decoder handling of interlaced files

7. Decoder handling of palettes

If PLTE is present in a PND datastream, the new palette is used in displaying the image defined by the PND; if no IDAT chunk is present and the image type is PNG indexed-color, then the image is redisplayed using the old pixel samples as indices into the new palette.

If a frame contains two or more images, the PLTE chunk in one image does not affect the display of the other, unless one image is a PND without a PLTE chunk, that has been declared by the DHDR ID field to depend on the other.

Note that a composite image consisting only of indexed-color images should not be assumed to contain 256 or fewer colors, since the individual palettes do not necessarily contain the same set of colors. Encoders can supply a gPLT chunk with a reduced global palette, to help decoders build an appropriate palette when necessary.

8. Security Considerations

An infinite or just overly long loop could give the appearance of having locked up the machine, as could an unreasonably long inter-frame delay or a misplaced SYNC chunk. Therefore a decoder should always provide a simple method for users to escape out of a loop or delay, either by canceling the MNG entirely or just proceeding on to the next SEEK chunk.

The gPLT chunk contains a "name" field that might be printed or displayed as text by some applications. As with the tEXt chunk, any non-printable characters in the gPLT "name" field, especially the ESC character, should not be displayed directly.

No known additional security concerns are raised by this format.

Detection of corrupted file transfers can be improved even beyond that available in PNG by using the MHDR max_chunk_size field to determine whether any chunk length (except for that of MHDR itself, which has a known length that can be checked) is unreasonably large.

9. Appendix: Examples

9.1. Example 1: Simple movie

\211 M N G \r \n ^z \n  # MNG signature
MHDR 720 468 720 468    # width and height
     20 65536  # 20 frames, max chunk length = 65kbytes
     30  3 60  # 10 frames per second, duration 60 ticks
     77000  8  # default gamma is 0.77, max_bit_depth 8
     3         # max 3 samples per pixel (color type 2)
     0         # not OK discard
     000000    # Six reserved bytes
tEXtTitle\0Sample Movie

SEEK 0 n1
IHDR 720 468 8 2 0 0 0   # DEFI 0 is implied
IDAT ...
IEND 


DHDR 0 1 1 20 30 100 220   # A PNG-delta frame
IDAT ...       # The IDAT gives the 20x30 block
DEND           # of deltas

DHDR 0 1 1 20 30 102 222     # Another PNG-delta frame
IDAT ...       # This time the deltas are in a 20 x 30
DEND           # block at a slightly different location

SEEK n1 n2      # Ok to restart here because a
                # complete PNG frame follows
IHDR 720 468 ...
IDAT ...
IEND

DHDR 0 1 1 720 468 0 0     # Another PNG-delta frame
IDAT ...   # The entire 720x468 rectangle changes
DEND       # this time.

SEEK n2 0
MEND            # end of MNG datastream

9.2. Example 2: Single composite frame

\211 M N G \r \n ^z \n # MNG signature
MHDR 1024 512 500 500 # width, height
     1 8192   # nframes, maxchunklen
     1 0 0    # frame duration can be zero since there's
              # only one frame but ticklength must be nonzero
     100000   # default gamma value is 100000 (gamma=1.0)
     16 1     # max depth 16, max 1 sample per pixel
     0        # not ok to discard
     000000   # Six reserved bytes
BACK 0 0 64 64 192   # sky blue background
FRAM                 # composite frame, 1024 x 512
LOCA 0 6 6           # Location of first image
                     # DEFI 0 is implied
IHDR 500 500 16 0 .. # A 16-bit graylevel image
gAMA 50000           # gAMA chunk takes precedence over
IDAT ...             # the default gamma value
IEND                 # End of image
                     # SHOW 0 0 is implied

LOCA 0 518 6         # Location to display a modified image.
DHDR 0 1 3 0 0 0 0  # reload image 0 and modify it
INHR gAMA 0 tEXt 3 faLT 0 IDAT 4 # establish chunk order
tEXtComment\0The faLT chunk is described in ftp://swrinde....
faLT ...             # Apply pseudocolor to basis image
DEND                 # End of image

LOCA 0 900 400       # Overlay near lower right-hand corner
IHDR 101 101 2 3 ... # Image 0 is redefined, but this does
                     # not affect the images already on screen
gAMA 50000           # We need a new gAMA because
PLTE ...             #    this is not a PND datastream
tRNS ...             # It's transparent (maybe a logo)
IDAT ...             # Note that the color type can differ
IDAT ...             #    from that of the other images.
IEND                 # End of image

ENDF                 # End of composite frame
MEND                 # End of MNG datastream

9.3. Example 3: Movie with sprites

\211 M N G \r \n ^z \n  # MNG signature
MHDR 512 512 512 512  # Start of MNG datastream
     0 0          # nframes, maxchunklen undefined
     30 3 3000   # 10 frames/sec, not more than 100 sec
     50000 8     # default_gamma 0.5, max_bit_depth 8
     1           # max samples per pixel
     0           # not ok to discard
     000000      # Six reserved bytes
FRAM             # First frame
DEFI 1           # Define image 1
                 # Location for image 1 is (0,0)
IHDR 512 512 ... # it's a full-display PNG image
etc              # chunks according to PNG spec
IEND             # SHOW 1 1 is implied by DEFI 1

DEFI 2           # Define image 2
LOCA 300 200     # Location for image 2
IHDR 32 32 ...   # It's a small PNG
gAMA 50000
IDAT ...
DEND             # IEND is omitted
ENDF             # end of frame

FRAM             # Second frame
                 # New location for image 1 is still 0,0
SHOW 1 1         # Display image 1 from previous frame
LOCA 1 10  5     # New (delta) location for image 2
SHOW 2 2         # Retrieve image 2 from previous frame,
CLON 2 3         # make another copy of it as image 3
LOCA 0 400 500   # Location for image 3
DHDR 3 1 3 0 0 0 0      # Modify image 3
tRNS ...         # Make it semitransparent
DEND             # SHOW 3 3 is implied by DHDR
ENDF             # End of second frame

FRAM             # Next frame (repeat this FRAM-ENDF
                 #   sequence with different locations to
                 #   move the images around)
                 # New location for image 1 is still 0,0
SHOW 1 1         # Display image 1 from previous frame
LOCA 1 10 5      # New (delta) location for image 2
SHOW 2 2         # Show image 2 from previous frame,
LOCA 1 5 -2      # New location for image 3
SHOW 3 3         # Show image 3
ENDF               # End of frame

FRAM           #  Another frame
etc.
ENDF 
etc.               # More frames
MEND               # End of MNG datastream

9.4. Example 4: "Fading in" a transparent image

\211 M N G \r \n ^z \n # MNG signature
MHDR 64 64 64 64  # width, height
     16 8192      # nframes, maxchunklen
     30 6 140  # tick length, frame and total duration
     50000     # default gamma
     8 1       # max bit depth 8, 1 sample per pixel
     0         # not ok to discard
     000000    # Six reserved bytes
BACK 1 0 192 192 192 # "browser gray" default background
DEFI 1
IHDR ...    # PNG header
PLTE ...
tRNS ...    # Entries are zero for the transparent color
            # and 16 for the nontransparent ones.  They
            # will be barely visible)
IDAT ...
IEND
LOOP 0 0 15
DHDR 1 1 3 0 0 0 0 
fADE 2 16   # Add 16 to alpha for all nontransparent
DEND        # colors.
ENDL 0      # Repeat loop.  After 15 iterations, the
            # opaque colors will end up with alpha=255
            # and the transparent ones will still be 0.
dURA 60     # Hold the last frame for at least 60 ticks
SHOW 1 1    # (2 sec).  Applications might show it longer,
            # or they might ignore dURA; it's ancillary.
MEND        # end of MNG

9.5. Example 5: Storing three-dimensional images

\211 M N G \r \n ^z \n # MNG signature
MHDR 150 150 150 150 24  # width, height, nframes
     65000       #  maxchunklen (doesn't have to be 2^n)
     30 0 0      # tick length, duration (can be zero)
     100000      # default gamma (voxel data is linear)
     16 1        # max bit depth 8, 1 sample per pixel
     0           # not ok to discard
     000000      # Six reserved bytes

tEXtTitle\0Weather modeling results
tEXtComment\0The pcAL, xsCL, ysCL, zsCA, and tsCL chunks
 in this file are written according to the PNG Sci-vis
 chunks specification version 19960921 available at
 ftp://swrinde.nde.swri.edu/pub/png-group/documents/

xsCLkilometers\0 0\0 150 # sci-vis "xsCL" chunk
ysCLkilometers\0 0\0 150 # sci-vis "ysCL" chunk
zsCAHeight (kilometers)\0 0\0 15
tsCLTime (hours)\0 0\0 24 # see proposed sci-vis chunks
pcAL 0 2 Degrees Celsius\0 0\0 45  # sci-vis "pcAL" chunk
SAVE
SEEK 0 3588720    # 3588720 bytes to the next SEEK chunk

FRAM             # initial composite image
IHDR 150 150 16  # width, height, bit depth for top layer
     0 0 0 0     # color, comp, filter, interlace
IDAT ...
IEND             # no DEFI chunk, so it's image 0

DHDR 0 1 0       # source=0, PNG, pixel subtraction,
     150 150 0 0 # block is entire image
IDAT ...     # IHDR is omitted; everything matches top
DEND         # IEND is also omitted

etc.   # repeat DHDR through DEND 148 more times
ENDF   # end of first block
SEEK 3588720 4621885

etc.   # Repeat SEEK through ENDF 19 more times

SEEK 2285321 0

MEND   # end of MNG

9.6. Example 6: Tiling

\211 M N G \r \n ^z \n  # MNG signature
MHDR 1024 768 128 64 ... # Start of MNG datastream
FRAM
LOCA 0 0 -64  # set up an offscreen copy of the tile
DEFN 1        # give it ID==1, don't show it
IHDR 128 64 ... # immediately
PLTE ...
IDAT ...      # Nothing will be displayed because it's
IEND ...      # outside the 1024 by 768 composite frame
              # and because we used DEFN instead of DEFI

LOOP 0 0 12   # Y loop -- make 12 rows of tiles
LOCA 1 0 64   # move the first copy down 64 rows
SHOW 1 1      # display it
CLON 1 2      # create a second copy of the tile

LOOP 1 0 7    # X loop - 7 additional columns
LOCA 1 0 128  # move it to the right 128 columns
SHOW 2 2      # use the second copy
ENDL 1

ENDL 0
ENDF
MEND

9.7. Example 7: Scrolling

\211 M N G \r \n ^z \n  # MNG signature
MHDR 512 256      # width and height on screen
     512 3000     # max_image must accommodate the
                  # largest stored image
     3257        # max no of frames
     32000       #  maxchunklen
     30 1 3257   # tick length, duration, total dur. 
     50000       # default gamma
     4 1         # max bit depth 4, 1 sample per pixel
     0           # not ok to discard
     000000      # Six reserved bytes
FRAM
DEFN 1            # Define image 1 but don't display now
LOCA 0 0 256      # initially it's offscreen, just
                  # below the 512 by 256 window
IHDR 512 3000 ... # A PNG datastream containing the
PLTE ...          # text (or whatever) to be scrolled
IDAT ...
IEND

DEFI 2
IHDR 512 256 ... # A PNG datastream containing some kind
PLTE ...         # of alpha-blended border that is
tRNS ...         # transparent in the center
IDAT ...
IEND

ENDF

LOOP 0 0 3256
FRAM
LOCA 1 0 1 # Jack image 1 up one scanline, 3256 times
SHOW 1 1   # It ends up just above the 512 by 256 window
           # The border does not move (LOCA 0 0 0 implied)
SHOW 2 2   # Overlay the transparent border
ENDF 
ENDL 0

MEND

9.8. Example 8: Converting a GIF animation to MNG

begin
    write MHDR chunk
    write BACK chunk
    write SAVE chunk
    write CLRB chunk
    
    for subimage in gif89a file do
       case gif_disposal_method in
    
          0: /* (undefined) */
             write <image>
    
          1: /* (keep) */
             write NOCL chunk
             write <image>
    
          2: /* (restore background) */
             write <image>
             write CLRB chunk
    
          3: /* (restore previous) */
             write SVCF 1 width height ... chunk
             write <image>
             write NOCL chunk
             write SHOW 1 1 chunk

       endcase
    endfor
    write MEND chunk
end

Where "<image>" represents a PNG or PND stream containing the
GIF frame converted to PNG format.  (Caution: you might have to pay
royalties to actually use your converter program or to give it to
anyone else).

Of course, this can be modified to write dURA chunks when needed.


10. Credits

Contributors' names are presented in alphabetical order:

Mark Adler,
madler@alumni.caltech.edu
Thomas Boutell,
boutell@boutell.com
John Bowler, jbowler@megamed.com
Christian Brunschen,
cb@df.lth.se
Glen Chapman, glenc@clark.net
Adam M. Costello,
amc@cs.berkeley.edu
Lee Daniel Crocker,
lee@piclab.com
 Peter da Silva, peter@starbase.neosoft.com

Andreas Dilger,
adilger@enel.ucalgary.ca
Oliver Fromme,
fromme@rz.tu-clausthal.de
Jean-loup Gailly, gzip@prep.ai.mit.edu
Chris Herborth, chrish@qnx.com
Alex Jakulin, alex@hermes.si
Neal Kettler,
kettler@cs.colostate.edu
Tom Lane, tgl@sss.pgh.pa.us

Alexander Lehmann,
alex@hal.rhein-main.de
Chris Lilley,
chris@w3.org
Dave Martindale, davem@cs.ubc.ca
 Carl Morris, msftrncs@htcnet.com
Owen Mortensen, ojm@csi.compuserve.com
Keith S. Pickens, ksp@swri.edu
Glenn Randers-Pehrson,
glennrp@arl.mil or randeg@alumni.rpi.edu
Greg Roelofs, newt@pobox.com

Willem van Schaik,
gwillem@ntuvax.ntu.ac.sg
Guy Schalnat
Paul Schmidt, pschmidt@photodex.com
Smarasderagd, smar@reptiles.org
Thomas R. Tanner, ttehtann@argonet.co.uk
Guido Vollbeding, guivol@esc.de
Tim Wegner, 71320.675@compuserve.com
Alaric B. Williams, png@abwillms.demon.co.uk


Trademarks:

Macintosh is a trademark of Apple Computer, Inc.
Windows 3.1 is a trademark of Microsoft Corporation.


Author's Address

Glenn Randers-Pehrson


U.S. Army Research Laboratory


ATTN: AMSRL-WM-TD


Aberdeen Proving Ground, MD 21005-5066

Phone: (410) 278-6554

EMail: glennrp@arl.mil or randeg@alumni.rpi.edu

End of MNG Specification.  Expires 28 March 1997